1. Field of the Invention
The present invention relates to an apparatus for detecting pressure in a cylinder of an internal combustion engine, which is used to measure information on the pressure in the combustion cylinder of an internal combustion engine.
2. Description of Related Background Art
In general, the pressure in a combustion cylinder is measured in order to determine the state of combustion of an internal combustion engine and the operation of each cycle of rotation. A piezoelectric cylinder pressure sensor is ordinarily used as a cylinder pressure sensor in such a pressure measurement. In such a sensor, the pressure in the cylinder is directly or indirectly applied to a piezoelectric element which generates electric charges in response to the pressure, and a charge amount corresponding to the applied pressure is then output. FIG. 1 shows an example of a piezoelectric cylinder pressure sensor. In the diagram, reference numeral 11 denotes piezoelectric elements; 12 indicates an electrode which is sandwiched between the two piezoelectric elements 11 and leads an output signal to a lead wire 13; and 14 is a casing which covers the internal structural parts of the sensor. The sensor has a ring-like shape and is placed between a cylinder head 2 forming an upper wall of a combustion cylinder of an engine and a spark plug 3, as shown in FIG. 2. The pressure in the combustion cylinder is propagated through the spark plug to the piezoelectric elements 11 of a piezoelectric cylinder pressure sensor 1, whereby charges corresponding to the cylinder pressure are generated.
Since an output signal corresponding to the cylinder pressure of the sensor represents a charge amount, the charge amount needs to be converted into the value of a voltage which can readily be electrically processed. For this purpose, hitherto, a charging amplifier has generally been used as means for converting the charge amount into the voltage value. FIG. 3 shows a fundamental circuit diagram of a charging amplifier. In FIG. 3, reference numeral 41 denotes an operational amplifier and 42 is a capacitor. An output of the sensor 1 is input to an inverting input terminal of the operational amplifier 41. The capacitor 42 is connected between the inverting input terminal of the operational amplifier 41 and an output terminal thereof. A non-inverting input terminal of the operational amplifier 41 is connected to the ground. The output of the operational amplifier 41 is controlled so as to equalize the levels of the voltages at the inverting and non-inverting input terminals. Accordingly, when an electric charge value Q from the sensor 1 is input to the operational amplifier 41, the amplifier operates to charge the capacitor 42 with the same amount as the charge value Q.
Therefore, assuming that the electrostatic capacity of the capacitor 42 is set at C, the voltage of V=Q/C is output from the operational amplifier 41. Since the charge amount Q is proportional to the cylinder pressure, the output voltage V of the operational amplifier 41 has a value corresponding to the pressure in the cylinder. During the operation of the engine, a combustion pressure signal as shown in FIG. 4 is output.
However, since the foregoing charging amplifier is of the type in which the charge amount is directly converted into the voltage value by the capacitor and the electrostatic capacity of the capacitor 42 is set to a small value in accordance with the electrostatic capacity of the piezoelectric elements 11 of the sensor 1, the following problem is encountered. If charges other than the cylinder pressure signal move, that is, if leakage currents flow through the sensor 1, the output signal line thereof, the input section of the charging amplifier, or the like, or if an input bias current flows through the operational amplifier 41 or the like, the output voltage of operational amplifier 41 fluctuates and hence the cylinder pressure cannot be accurately measured.
Further, the pressure/charge amount converting characteristics in the sensor 1 vary in accordance with temperature changes, and so-called pyro effect acts to cause charges to be generated in accordance with changes in the temperature of the piezoelectric element. In particular, when the sensor is attached near the combustion chamber as shown in FIG. 2, a problem is encountered in that the temperature change which occurs each cycle due to increases in the temperature of the cylinder head 2 or spark plug 3 or any temperature transfer in the combustion chamber is large. This means the output signal waveform of the charging amplifier is greatly influenced by such temperature changes and the cylinder pressure cannot be accurately measured. An additional problem is that, even if attempts are made to provide for temperature compensation by certain means, the construction of such means will inevitably be complicated because the output charge amount in the circuit of the charging amplifier is directly converted into the voltage value by the capacitor 42.
There is still another problem in that although the output voltage of the charging amplifier 41 changes in accordance with variations in cylinder pressure, it cannot represent the absolute pressure value of the pressure in the cylinder.